Vitreous humour (British spelling) or Vitreous humor (U.S. spelling) is the clear aqueous solution that fills the space between the lens and the retina of the vertebrateeyeball. The solution is 99% water, but has a gelatinous viscosity two to four times that of water. The remaining solutes include salts, sugars, phagocytes, and a network of collagen fibres. The phagocytic cells are present to remove unwanted debris in the visual field. The primary purpose of the vitreous humour is to provide a cushioned support for the rest of the eye, as well as a clear unobstructed path for light to travel to the retina.

The collagen fibres of the vitreous are held apart by electrical charges. With aging, these charges tend to reduce, and the fibres may clump together. Similarly, the gel may liquify, a condition known as syneresis, leading to cells and other organic clusters to float freely within the vitreous. These commonly lead to floaters which are perceived in the visual field as spots or fiberous strands. Floaters are generally harmless, but the sudden onset of recurring floaters may signify a posterior vitreous detachment (PVD) or other diseases of the eye.

The metabolic exchange and equilibration between systemic circulation and vitreous humor is so slow that vitreous humor is sometimes the fluid of choice for postmortem analysis of glucose levels or substances which would be more rapidly diffused, degraded, excreted, or metabolized from the general circulation.